Chapter 1 Introduction
1.4 Thesis Organization
In chapter 1, we describe our background and motivation of this study.
In chapter 2, we used different solvents, xylene and chloroform, to dissolve P3HT, and then used spin-coating technique to form organic semiconductor layer. Afterwards, we investigated electrical characteristics of P3HT OTFTs. In addition to studying different solvents, we prepared different weight percentages of P3HT in chloroform, 0.1% , 0.3%, 0.8% and 2.0%. Afterwards, we investigated electrical characteristics of P3HT OTFTs.
In chapter 3, we treated OTFTs with O2, N2 and H2O deliberately to clarify the correlation between electrical characteristics of P3HT OTFTs and the exposed ambient. Additionally, we investigate the behavior of P3HT OTFTs during stress measurements.
In chapter 4, we employ different electrode materials, such as Ti, Ni, Pt and Au, to check it can form ohmic contact between source/drain electrodes and the organic semiconductor or not.
Next, we adjust adhesion/contact thickness ratio to check it can affect the contact resistance between source/drain electrodes and the organic semiconductor or not.
In chapter 5, we will describe the conclusions and the future work.
Figure 1-1:Semilogarithmic plot of the highest field-effect mobility(µ)
reported for OTFTs fabricated from the most promising polymeric and oligomeric semiconductors versus year from 1986 to 2000.[1](a)
(b)
Figure1-2:Schematic of operation of organic thin film transistor, showing a
lightly p-doped semiconductor: + indicates a positive charge in semiconductor ; (-)indicates a negatively charge counterion (a) no-bias (b) accumulation mode (c) depletion mode (d) non-uniform charge density (e) channel pinch-off [3] (continue)(c)
(d)
Figure1-2:Schematic of operation of organic thin film transistor, showing a
lightly p-doped semiconductor: + indicates a positive charge in semiconductor ; (-)indicates a negatively charge counterion (a) no-bias (b) accumulation mode (c) depletion mode (d) non-uniform charge density (e) channel pinch-off [3] (continue)(e)
Figure1-2:Schematic of operation of organic thin film transistor, showing a
lightly p-doped semiconductor: + indicates a positive charge in semiconductor ; (-)indicates a negatively charge counterion (a) no-bias (b) accumulation mode (c) depletion mode (d) non-uniform charge density (e) channel pinch-off [3]Table1-1:Highest field-effect mobility(µ) values measured from OTFTs as
reported in the literature annually from 1986 through 2000.[1]Chapter 2
OTFTs Fabricated by Different Solvents and Weight Percentages of P3HT
2.1 Introduction
Lately, organic materials such as pentacene and poly (3-alkylthiophene) have attracted considerable attention due to their potentials for semiconductor electronics. In particular, the characteristics of P3AT thin film transistors (TFTs) have made such devices feasible for applications requiring large-area coverage, mechanical flexibility, and low overall cost.
There are four methods to form organic semiconductor film: (a) solution-processed deposition, (b) vacuum evaporation, (c) electro-polymerization, (d) Langmuir-Blodgett Technique [4]. Recently, many researchers extensively use solution-processed deposition to fabricate organic semiconductor film. For solution-deposited organic semiconductor film, one kind of the organic semiconductor material such as poly (3-alkylthiophene) are dissolved in solvent such as chloroform, and then the dissolved organic material are deposited onto the surface of substrate using spin-coating, dip-coating or drop-casting method. Among these three methods of casting P3HT thin film, the best way is dip-coating and it has been shown that the field-effect mobility is as high as 0.2 cm2/Vs [5]. Nevertheless, devices made by dip-coating technique have a good mobility, but the dip-coating method can not be applied for coverage of a large area.
Therefore, in all of our experiments, we used spin-coating technique as a key process of organic layer deposition.
Among a lot of organic semiconductor materials, we use poly (3-hexylthiophene) or P3HT
as the semiconducting layer. P3HT has many potential advantages to serve as the active layer of field-effect transistors. First, P3HT is a well-known organic semiconducting polymer and has shown the field-effect mobility from 10-4 cm2/Vs in 1988 to 0.2 cm2/Vs in 2003[1], [4], [6].
Second, P3HT has high solvent selectiveness, which can dissolve in toluene, xylene chloroform and so on. Third, P3HT is solution processed; therefore it can be processed by spin coating.
However, the performance of P3HT thin film transistors can be affected by: (1) the regioregularity of the P3HT, (2) solvents, (3) P3HT weight % in solvents, and (4) different deposition methods, such as spin-coating, dip-coating, or drop-casting [7].
In this chapter, we used different solvents, xylene and chloroform, to dissolve P3HT, and then used spin-coating technique to deposit organic semiconductor layer. Afterwards, we measured the electrical characteristics of P3HT OTFTs formed by different solvents, such as mobility, threshold voltage and on/off ratio. In addition to study different solvents, we also prepared different weight percentages of P3HT in chloroform, i.e. 0.1%, 0.3%, 0.8% and 2.0%.
Afterwards, we investigated the electrical characteristics of P3HT OTFTs including mobility, threshold voltage and on/off ration etc., with different weight percentages of P3HT.
2.2 The Molecular Structure of P3HT
The structure of the polymer chain of P3HT is shown in Fig 2-1. The 3-alkyl substituents can be incorporated into a polymer chain with two different regioregularities: head to tail (HT) and head to head (HH) [7], [8].
R represents the alkyl side chain (C6H13, for hexylthiophene), which allows them to be dissolved in solvents, such as xylene or chloroform. This solution processability enables simple film deposition, one of the major attractions of conjugated polymers. A regiorandom P3HT
consists of both HH and HT 3-hexyllthiophenes in a random pattern while a regioregular P3HT has only one kind of 3-alkylthiophene, either HH or HT. The position and direction of the side chain in the diagram shows a very highly ordered system. This type of order is known as regioregularity and has been shown to give much higher field-effect mobility values over regiorandom (disordered side chains) material [9]. Most interestingly, these polymers have been shown to have very different properties from their corresponding regiorandom polymers, such as smaller band gaps, better ordering and crystallinity in their solid states, and substantially improved electroconductivities. When regioregular P3HT consisting of 98.5% or more head-to-tail (HT) linkages was used to fabricate FETs, a dramatic increase in mobility was observed relative to regiorandom poly-3-alkylthiophenes [10]. In our experiments, regioregular P3HT (HT regioregularity of 98.5%) and high grade solvents, xylene and chloroform, were purchased from Aldrich Chemical Company. We did not perform further purification or sublimation to these chemicals.
2.3 Fabrication of Organic Thin Film transistors
2.3.1 Process Flow of P3HT OTFTs Fabrication
The P3HT TFTs use a bottom-contact structure fabricated on silicon substrate as shown in Fig 2-2. The process flow of P3HT device is as following.
At first, an n-type bare silicon wafer was cleaned by the standard RCA cleaning process.
After that, phosphorus atoms were diffused into an n-type silicon wafer by POCl3 to form a common gate electrode. After diffusing, we used dilute HF to remove SiO2 and measured its sheet resistance (3~4Ω/□) .Before the insulating layer of silicon dioxide was deposited, the n+
silicon wafer must be cleaned again by the standard RCA cleaning process. Then a silicon
dioxide layer is deposited by PECVD using TEOS source and O2 gas at 350℃. Afterwards, source and drain regions were defined through the photo lithography process followed by thermal evaporation steps of 20-nm thick Ti as an adhesion layer and 100-nm thick Pt as a contact material. Later, the wafer was then immersed in acetone to lift-off the photo resist/metals and to form the source/drain regions. The samples after S/D patterning were treated with 3 minute IPA cleaning, 5 minute D.I water cleaning. Next, oxide surfaces were treated with hexamethyldisilazane (HMDS) to improve the adhesion between the polymer chain and oxide surfaces. Then we prepared different solvents, xylene or chloroform, to dissolve P3HT. P3HT solution was filtered by a 0.2-µm pore-size PTFE filter and then spun onto the wafer surface. The detailed spin-coating parameters is 200 RPM for 10S as step one, 500 RPM for 25S as step two and 2000 RPM for 25S as step three. Finally, the samples were cured at 120℃ for 3 minute. The process flow of P3HT based OTFTs is summarized in Fig2-3.
2.3.2 Modification of Oxide Surface
Oxide surfaces were treated with hexamethyldisilazane (HMDS) to improve the adhesion between polymer chain and oxide surfaces. Modification of the substrate surface prior to deposition of regioregular P3HT has also been found to influence the film morphology. For example, treatment of SiO2 with hexamethyldisilazane (HMDS) replaces the hydroxyl groups at the SiO2 surface with methyl or alkyl groups. The apolar nature of these groups apparently attracts the hexyl side chains of P3HT, favoring lamellae with an edge-on orientation (Fig2-4) [11]. According to the reference [11], the mobility of OTFTs with an edge-on orientation P3HT film is higher than that with a face-on orientation.
2.3.3 The Layout of Bottom-Contact OTFT
We used two kinds of layout in the fabrication of OTFTs, i.e. linear type and finger type as shown in Fig2-5.An interdigitated geometry as shown in Fig2-5 (b) for the source/drain contacts is chosen to minimize the device area and the associated gate to source/drain leakage current. The channel length (L) of the linear type layout is in a range of 10~50μm and the channel width (W) is in a range of 300~500μm. The channel length of the finger type is in a range of 10~50μm and the channel width is in a range of 1000~10000μm.
2.4 Electrical Characteristics of P3HT OTFTs
2.4.1 Measurement
Current-voltage characteristics of OTFT were measured in the air with a semiconductor parameter analyzer HP4156. All measurements were carried out in an electrically shielded box.
The drain-source current IDS was measured as a function of the drain-source voltage VDS to observe the FET-like characteristics. And IDS was measured as a function of the gate voltage VG
at a small drain-source voltage, which was constructed to determine the gate bias modulation of the FET conductive channel.
Three parameters were extracted from the experimental I-V curves and are: (1) the transistor threshold voltage (VT), (2) the current modulation (the ratio of the current in the accumulation mode over the current in the depletion mode, also referred to ON-OFF current ratio), and (3) the field effect mobility (μFE). The detailed extraction method will be discussed in the following section.
2.4.2 Threshold Voltage and OFF Current Definition
Inorganic semiconductors, such as Si or Ge, can be operated in three modes: depletion mode, accumulation mode and inversion mode. With regards to organic semiconductors, such as P3HT OTFTs, they can not be operated in the inversion mode. Therefore, P3HT OTFTs were turned ON in the accumulation mode (VG<0, see Fig1-2(b)) and were turned OFF in the depletion mode (VG>0, see Fig1-2(c)).
Because P3HT OTFTs are normally on devices, we define a so-called OFF current when the current is smaller than a certain value, and the gate voltage (VG) corresponding to this current is called threshold voltage (Vth). Fig2-6 shows a plot of normalized drain-source current vs. gate voltage. Based on this figure, we defined the normalized OFF current is 10-12 Amp and thus the OFF current of a specified transistor is 10-12 *W/L. The magnitude of OFF current with different channel length and different channel width are listed Table2-1
2.4.3 The Extraction method of Mobility
P3HT OTFT is like a p-channel FET. Therefore, the linear regime field effect mobility can be obtained by the calculation described below. At low drain voltage (VD), source-drain current (IDS) increases linearly with VD (linear regime) and is approximately determined from the following equation (2-1):
where L is the channel length, W is the channel width, Ci is the capacitance per unit area of the insulating layer, Vth is the threshold voltage, and μ is the field effect mobility, which can be calculated in the linear regime from the transconductance,
V const i D
by plotting IDS versus VG at a constant low VD, with -VD << -(VG - VT), and equating the value of the slope of this plot to gm. We can compute the linear regime mobility from equation 2-2
2.5 OTFTs Fabrication by Different Solvents
2.5.1 Experiment Detail
The detailed process flow of P3HT OTFTs fabrication was described in section2.3.1. The different portion is that we prepared different solvents including xylene and chloroform to dissolve P3HT material. Specifically, the P3HT films were deposited from a solution of 0.3%
P3HT in xylene or 0.3% P3HT in chloroform. And then the P3HT solution was filtered by a 0.2-µm pore-size PTFE filter and spun onto the wafer surface.
2.5.2 Result and Discussion
2.5.2.1 Physical properties of spin-on P3HT film
We used atomic force microscope to observe the surface morphology and topography of the deposited P3HT film. Fig2-7 exhibits the surface morphology of the deposited P3HT film with different solvents. Fig2-7(a) shows that many clusters of undissolved P3HT powder, despite being filtered, still can be observed, implying that xylene is not a good solvent for P3HT. But from Fig2-7(b), we can not find apparentclusters of undissolved P3HT powder. It follows that chloroform is a good solvent to dissolve P3HT material. Additionally, no apparent grain or grain-boundary structure was found in the AFM photograph because the P3HT thin film is a long-chain polymer.
2.5.2.2 The anomalous gate leakage current effect from xylene solution
Current-voltage characteristics of OTFTs were measured in the air with a semiconductor parameter analyzer HP4156. Fig2-8 illustrates source current (IS) and gate leakage current (IG) versus gate voltage where the P3HT OTFT was fabricated by 0.3% xylene solution. The OTFT was turned ON, and the “ON-current” was larger than the gate leakage current by one order. The gate leakage current was comparable to the source current when the device is nearly turned OFF, and finally the gate leakage current dominated the drain current. Due to anomalous gate leakage effect, we cannot measure the ideal I-V characteristics. As a result, the ON-OFF ratio would be affected by anomalous gate leakage current.
Fig2-9(a) shows source current versus drain voltage, where the P3HT OTFT was fabricated by 0.3% xylene solution. Fig2-9(b) shows source current versus drain voltage, where the P3HT OFTF was fabricated by 0.3% chloroform solution. Because of anomalous gate leakage current, the source current at zero bias was above 10-7 Amp as shown in Fig2-9(a). If the solvent was changed from xylene to chloroform, we could not observe apparent anomalous gate leakage current. From Fig2-9(b), the source current at zero bias was below 10-8Amp.Therefore, the anomalous gate leakage current was suppressed by chloroform solution.
Because the anomalous leakage current is comparable to the source current of the OTFT with small W/L ratio, the result would be incorrect. Therefore, as the P3HT OTFT was fabricated by 0.3% xylene solution, the anomalous gate leakage current not only influenced the magnitude of source current at zero drain bias, but also the ON-OFF ratio with small W/L. From Fig2-10, if the P3HT OTFT was fabricated by 0.3% xylene solution, ON-OFF ratio was dependent on W/L ratio. But the solvent was changed from xylene to chloroform, the anomalous gate leakage current was suppressed. From Fig2-10, if the P3HT OTFT was fabricated by 0.3% chloroform solution, ON-OFF ratio was independent on W/L ratio.
2.5.2.3 The correlation between field-effect mobility of P3HT OTFT and solvent
The choice of solvents has a very significant impact on the field-effect mobility of P3HT OTFTs. In a recent publication, Bao et al. [12] observed that chloroform was used as a solvent and P3HT organic semiconductor layer was deposited by spin-coating, the field-effect mobility of P3HT OTFT is about 10-3 cm2/Vs. Xylene was used as a solvent and P3HT organic semiconductor layer was deposited by spin-coating, the field-effect mobility of P3HT OTFT is about 10-4 cm2/Vs, as shown in Table2-2. From Fig2-11, the field-effect mobility of P3HT OTFT which was fabricated by xylene is about 10-3 cm2/Vs and the field-effect mobility of P3HT OTFT which was fabricated by chloroform solution is about 10-4 cm2/Vs. The conclusion was in consistent with the publication [12].
2.6 OTFTs Fabrication by Different Weight Percentages of P3HT
2.6.1 Experiment Detail
In this section, we prepared 0.1%, 0.3%, 0.8%, and 2.0% of P3HT in chloroform. And then the P3HT solution was filtered by a 0.2-µm pore-size PTFE filter and then spun onto the wafer surface. Next, we investigated the electrical characteristics of P3HT OTFTs, such as mobility, threshold voltage and on/off ratio, with different weight percentages of P3HT. Besides, we compared the performance of P3HT OTFTs which were fabricated by new and old P3HT material. New P3HT material, specifically “fresh” P3HT material, means that we purchased it from Aldrich chemical company and were used at once. Old P3HT, although purchased from the same company, had been opened and the remaining chemicals had been storing in air for a year.
2.6.2 Result and Discussion
2.6.2.1 Physical properties of spin-on P3HT film
We used atomic force microscope to observe the surface morphology and topography of deposited P3HT film. Fig2-12~Fig2-15 exhibits the surface morphology of the deposited P3HT film with different weight percentage of 0.1%, 0.3%, 0.8% and 2.0% of P3HT in chloroform. It was found that the deposited films by the low weight percentage of P3HT, such as 0.1% and 0.3%, were very smooth, but the deposited films by high weight percentage of P3HT, such as 0.8% and 2.0%, were very rough. When the weight percentage of P3HT as high as 2.0%, there were apparent pinholes in the deposited film. Table2-3 summarized the surface roughness of P3HT film with respect to weight percentage of P3HT in chloroform. The surface root-mean-square roughness of organic thin film deposited by 0.3% of P3HT is 8.24Å. That is much smoother than RMS roughness of organic thin film deposited in other weight percentages.
2.6.2.2 The bulk current effect from high weight percentage of P3HT
There are two current paths in organic semiconductor layer[13]. One is the channel current (Ich), it comes from source electrode (Pt) and goes through the accumulation holes and into drain electrode (Pt). Using established metal-oxide-field effect transistor (MOSFET) current-voltage relationships, the channel current can be written as:
D
for the linear regime, where L is the channel length, W is the channel width, Ci is the capacitance per unit area of the insulating layer, Vth is the threshold voltage, and μ is the field effect mobility.
Another leakage path is the bulk current (Ibk). It comes from source electrode (Pt) and goes through conductive layer which is above the accumulation holes and into drain electrode. The bulk current (Ibk) can be represented as
bk l VDS L
I =µ×W × × [Equation 2-4]
, where l is the organic semiconductor layer thickness [13].
The P3HT OTFTs were turned ON in the accumulation mode (VG<0, see Fig1-2(b)) and were turned OFF in the depletion mode (VG>0, see Fig1-2(c)). Fig2-16 shows a typical source current versus drain voltage plot at various gate voltages in both accumulation [Fig.2-16(a)] and depletion [Fig.2-16(b)] modes. Fig2-17 is the same. As the positive voltage increases, the source current decreases. It was shown that the device could be turned OFF. But from Fig2-18, IS versus VD curve in the depletion mode as a function of weight concentration of P3HT in chloroform, the devices which were fabricated by 0.8%, 2.0 % of P3HT can not be turned OFF. From two aspects of observation, it can be shown that the current is the bulk current. (1) As the organic semiconductor layer thickness increases, the source current increases. (2) As the drain voltage increases, the source current increases. These conclusions are consistent with Equation 2-4.
Either new or old P3HT material, the OTFTs fabricated by high weight percentage of P3HT such as 0.8% and 2.0% can not be observed the ideal IS-VG characteristics as shown in Fig2-19~Fig2-20 due to the bulk current effect. As a result, threshold voltage and ON-OFF ratio would be affected by the bulk current. Fig2-21 and Fig2-22 shows that threshold voltage as a function of weight concentration of P3HT in chloroform. Because of the bulk current effect there is a dramatic increase as weight concentration of P3HT is above 0.3%.
2.6.2.3 The correlation between the performances of P3HT OTFT and weight percentage of P3HT
For OTFTs fabricated by fresh P3HT material, the following phenomenon can be observed:
(1) Fig2-22 illustrates the dependence of threshold voltage and various weight percentages of
P3HT. The most appropriate wt% of P3HT is 0.1%-0.3%. (2) Fig2-23 illustrates that the field-effect mobility (weight %) dependence shows a maximum at 0.3%-0.8%. (3) Fig2-24 illustrates that ON-OFF ratio (weight %) dependence show a maximum at 0.1%-0.3%. OTFTs fabricated by 0.8 % of P3HT had a better mobility than the others, such as 0.1%, 0.3%, 2.0%, but they can not have an ideal IS-VG characteristic. Therefore, in order to acquire an OTFT with good mobility, high ON-OFF ratio as well as appropriate threshold voltage, the optimal weight percentage of P3HT would be 0.3%. For OTFTs fabricated by old P3HT material, the foregoing
P3HT. The most appropriate wt% of P3HT is 0.1%-0.3%. (2) Fig2-23 illustrates that the field-effect mobility (weight %) dependence shows a maximum at 0.3%-0.8%. (3) Fig2-24 illustrates that ON-OFF ratio (weight %) dependence show a maximum at 0.1%-0.3%. OTFTs fabricated by 0.8 % of P3HT had a better mobility than the others, such as 0.1%, 0.3%, 2.0%, but they can not have an ideal IS-VG characteristic. Therefore, in order to acquire an OTFT with good mobility, high ON-OFF ratio as well as appropriate threshold voltage, the optimal weight percentage of P3HT would be 0.3%. For OTFTs fabricated by old P3HT material, the foregoing